Ss. Spicer et al., EXPRESSION PATTERNS OF ION-TRANSPORT ENZYMES IN SPIRAL LIGAMENT FIBROCYTES CHANGE IN RELATION TO STRIAL ATROPHY IN THE AGED GERBIL COCHLEA, Hearing research, 111(1-2), 1997, pp. 93-102
Fibrocytes of the lateral wall function in conjunction with the stria
vascularis (StV) to mediate cochlear ion homeostasis. Age related chan
ges in the expression patterns of ion transport enzymes in spiral liga
ment fibrocytes were investigated to ascertain their relation to metab
olic presbyacusis in the gerbil. Immunoreactivity of fibrocytes for Na
,K-ATPase (Na,K), carbonic anhydrase isozyme II (CA) and creatine kina
se isozyme BB (CK) varied among and within cochleas from aged but not
from young gerbils. The variable immunostaining was related to the ext
ent and location of StV atrophy. Age-dependent degeneration and loss o
f Na,K in the StV occurred predominantly in the apex and lower base an
d hook of the cochlea, largely sparing more central regions. Immunosta
ining intensity for Na,K, CK, and CA in fibrocytes changed in relation
to declines in strial marginal cell Na,K initially showing upregulati
on followed by downregulation. Spiral ligament fibrocytes in cochleas
with more than two remaining normal turns often disclosed overexpressi
on of CK in regions of strial atrophy. Conversely, CA in such cochleas
was often increased in regions of normal StV adjacent to foci of atro
phic StV. Senescent cochleas with two or fewer functional turns genera
lly contained fibrocytes with diminished CK or CA immunoreactivity in
regions of atrophic StV but in isolated instances exhibited fibrocytes
with enhanced staining. Heightened staining for CK in type Ia fibrocy
tes underlying regions of complete or partial strial atrophy indicated
an increased metabolic demand in fibrocytes in response to strial ins
ufficiency. The findings provide further support for the role of spira
l ligament fibrocytes in cochlear fluid and ion homeostasis.